Murach, A. A., and T. Kasirenko. “Elliptic Problems With Boundary Operators of Higher Orders in Hörmander–Roitberg Spaces”. Methods of Functional Analysis and Topology, vol. 24, no. 2, June 2018, pp. 120-42, https://zen.imath.kiev.ua/index.php/mfat/article/view/690.